An accelerometer is one of the major sensors used in navigational systems, particularly inertial navigational systems, and on-board automotive safety control systems. Automotive examples of accelerometer use include various anti-lock braking systems, active suspension systems, and seat belt lock-up systems.
Generally, an accelerometer is a device which measures acceleration and, in particular, an accelerometer measures the force that is exerted when a moving body changes velocity. The moving body possesses inertia, which causes the body to resist the change in velocity. It is this resistance to a sudden change in velocity that is the origin of the force which is exerted by the moving body when it is accelerated. This force is proportional to the acceleration component in the direction of the movement, and therefore may be detected by an accelerometer.
In a typical accelerometer, a mass is suspended by two springs attached to opposite sides of the mass. The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the system undergoes a change in velocity in the direction of the springs' axis or perpendicular to the springs, axis, and therefore is accelerated in a particular direction, the spring mounted mass will at first resist the movement along that axis because of its inertia. This resistance to the movement, or delay in the movement, will force the springs to be temporarily either stretched or compressed. The tensile or compressive force acting on each spring is related to the product of the weight of the mass and the acceleration of the mass. The acceleration is then correspondingly determined by the change in velocity experienced by the mass.
Integrated circuit microaccelerometers having a proof mass suspended by pairs of piezoelectric microbridges are also known. An illustrative example of this type of accelerometer is disclosed in U.S. patent application Ser. No. 07/304,057 to Chang et al entitled "Resonant Bridge Two-Axis Accelerometer". In a microaccelerometer of this type, a proof mass is suspended by at least one pair of piezoelectric microbridges. The pair of microbridges are attached to opposite ends of the proof mass along a common axis. The acceleration of the mass is determined by the change in force acting upon each piezoelectric bridge. This type of resonant microaccelerometer is attractive for precision measurements, because the frequency of a micromechanical resonant structure can be made highly sensitive to physical or chemical signals.
A shortcoming exists with regard to the manufacturing of these and other types of microaccelerometers. The microbridges from which the proof mass is suspended, are typically formed from extremely thin layers of material, generally silicon. These thin microbridges are extremely susceptible to damage if the proof mass is allowed to deflect excessively. Therefore it is desirable that the proof mass be permitted to deflect sufficiently to produce an adequate signal for measurement purposes, but not so much that it causes degradation to the microbridges or other components of the microaccelerometer. A common practice has been to sandwich the silicon wafer having the accelerometer components, between two other wafers of a compatible material, wherein there is a predetermined gap between the proof mass and each of the two surrounding wafers to permit the proof mass to deflect. The gap is in accordance with the design requirements for the sensor and permits the proof mass to deflect that predetermined distance before contacting the surrounding wafer, which prevents it from deflecting any further.
However, there are difficulties associated with the current methods for manufacturing microaccelerometers of this type wherein the device wafer is sandwiched between two wafers of a compatible material. One common method has been to electrostatically bond the silicon microaccelerometer device between two glass plates; each of the glass plates having a recess of appropriate height so as to permit the deflection of the proof mass. However, the mismatch in thermal coefficients of expansion between the silicon wafer and glass wafers causes thermally induced stresses over the wide temperature range which the microaccelerometer must satisfactorily operate. This is particularly true if the microaccelerometer is within an automobile environment. Also, glass is inherently difficult to machine, which is necessary for formation of the required recesses so as to permit deflection of the proof mass. For these reasons, this current practice of bonding the silicon microaccelerometer wafer between two glass plates is unacceptable.
An alternative method for forming the microaccelerometer has been to sandwich the silicon device between two silicon wafers, again each of the surrounding silicon wafers having an appropriately provided recess for deflection of the proof mass. The silicon wafers are then bonded together using conventional gold eutectic bonding techniques. However, this method, although it alleviates the problem of mismatched thermal coefficients, is also problematic. In order to initiate the formation of the eutectic bond, the wafer surfaces must be aggressively scrubbed together to ensure intimate contact between the surfaces. This is unacceptable once the microaccelerometer components have been formed on the intermediate silicon wafer, since this aggressive action is detrimental to the device components. Yet it is only after the components have been formed that the surrounding wafers can be bonded to the microaccelerometer, thereby making this method also unacceptable.
Therefore, what is needed is an accelerometer which avoids these shortcomings of the prior art. It would be desirable to provide a microaccelerometer wherein the bonds between the accelerometer and surrounding wafers are characterized by a relatively low stress level which do not result in unnecessary residual stress to the device. Further, it is desirable that such a microaccelerometer have means for facilitating accurate and precise control of the spacing between the bonded wafers, while also preferably incorporating stop means into its design to prevent excessive deflection of the proof mass. Lastly, it is preferable that such bonds within such an accelerometer be formed without degradation to those components, such as by forming at a relatively low temperature without any aggressive scrubbing type action.